The Efferent Innervation of the Ear: Variations on an Enigma

For more than forty years, the efferent supply to the mammalian ear provided by the olivocochlea (OC) bundle (Rasmussen 1946) has been an enigma. Rasmussen (1953) recognized that because of this pathway “the receptors would possess a double innervation which would be most interesting… particularly from the point of view of neural mechanisms of hearing” and initial studies soon implicated the OC bundle in auditory function. When the OC axons were transected, nerve endings near hair cells degenerated (Smith and Rasmussen 1963) and when they were stimulated, the total sensory activity of the auditory nerve was reduced (Galambos 1956). In the two decades following these fundamental observations, little insight into the significance of the OC system was obtained and some authors concluded that it served no auditory function (see Klinke and Galley 1974). Indeed, it was difficult to see how the efferent system could operate, since in mammals the majority of the auditory sensory nerve fibers connect with inner hair cells yet it is the outer hair cells which are the main target of the efferent innervation.